This invention relates to photoplotters and similar devices for exposing lines on a photosensitive surface by flashes of light produced by a flash lamp, and deals more particularly with such an apparatus and method wherein any one of a number of apertures of different size may be used with the flash lamp to control the width of the exposed line and wherein for any selected aperture the number of flashes used to expose each incremental length of the line may be set, and preferably varied, to control the quality of the resulting line.
The apparatus and method of the invention are usable in various instances wherein lines are exposed on a photosensitive surface by using a flash lamp and an associated aperture to project a plurality of overlapping light spots, usually circular shape, onto the photosensitive surface to synthesize a line. A well known application is a photoplotter wherein a sheet of material having a photosensitive surface is held to the support surface of a table while a photohead is moved in X and Y coordinate directions over the photosensitive surface to trace lines on the photosensitive surface. As this line tracing movement of the photohead occurs, a flash lamp in the photohead is flashed at such a rate that the spots projected onto the photosensitive surface overlap one another and expose the traced line.
Flash lamps have a maximum achievable flash rate at which they can be flashed; and generally in the case of a photoplotter, especially for small apertures, the maximum achievable speed of the plotter is such that when the plotter is driven at that speed the lamp cannot be flashed at a high enough rate to achieve an overlap of the projected light spots. This problem has been dealt with in the past by limiting the speed at which the photohead is driven relative to the photosensitive surface for any one of a group of differently sized apertures to one at which a suitable line can be exposed with the smallest aperture of the group, and the other apertures of the group are then fitted with filters to cause proper exposure of a line when any one of them is used as the selected aperture. That is, for a given group of apertures covering a given size range, the photohead was driven at the same limited maximum speed regardless of the aperture selected for use with the flash lamp.
The above mentioned prior art procedure of driving a photohead at the same limited maximum speed for any aperture selected from a given size group has the disadvantage of reducing the throughput of the plotter, since the photohead, except for the smallest aperture of the group, is driven at a maximum speed less than that imposed by the maximum achievable flash rate of the flash lamp. Also, in the prior art arrangement it is difficult to adust the photohead to photosensitive surfaces of different speed or sensitivity or to aging or replacement changes in the output of the flash lamp, and to otherwise control the exposure of the photosensitive surface, as to do so involves the replacement of the filters associated with the various apertures.
Also, the quality of the line exposed, as to the straightness of its edges and degree of exposure, depends on the number of spots projected or flashes generated for each increment of length of the exposed line. For circular spots the involved quanity may be stated in terms of flashes per spot diameter. For small width lines, that is lines synthesized from spots of 2 to 6 mils, lines of acceptable quality may perhaps be produced with as few as 31/2 to 5 flashes per spot diameter, while, when using spots of larger diameter, for example 160 mils, as many as 14 flashes per diameter may perhaps be required to produce acceptable line quality. In any event, the number of flashes per spot diameter has an effect on line quality and in known prior art photoheads it has been impossible to individually set or vary, for each aperture, the number of flashes produced per spot diameter to gain control over line quality.
Photoheads used for exposing lines are also commonly used to expose individual features on the photosensitive surface while the photohead is held stationary relative to the surface. Such individual features are commonly referred to as "pads" and often are of circular shape. In prior art photoplotters it has generally been necessary to provide one circular aperture of a given size for pad flashing and one aperture of the same size for line drawing as most or all of the line drawing apertures have filters associated with them and all of the pad flashing apertures are unfiltered.
The general object of the invention is to provide an apparatus and method for exposing lines which avoids the above mentioned drawbacks of known prior art photoplotters. More particularly, an object of the invention is to provide an apparatus and method for exposing lines on a photosensitive surface by flashes of light wherein the throughput of the apparatus and method are greatly increased by driving the photohead for each selected aperture at a maximum limit speed corresponding to operation of the flash lamp at or near its maximum achievable flash rate.
A further object of the invention is to provide an apparatus and method of the foregoing character which allows great control over the quality of a line exposed by the photohead by permitting the setting for each aperture of the number of flashes to be generated per spot diameter or other increment of line length with such setting preferably being variable to suit the photohead to changes in desired quality, to changes in the photosensitive surface to be exposed or to changes in other factors.
A still further object of the invention is to provide an apparatus and method of the aforegoing character wherein the need for filters associated with the apertures is eliminated, thus avoiding the expense of the filters and also permitting each line drawing aperture to be used as a pad flashing aperture.
Other objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings and the claims.